Increasing the efficiency of power generation plants is in progress in response to demands for reduction of carbon dioxide, resource conservation, and the like. Specifically, increasing the temperature of a working fluid of a gas turbine and a steam turbine, employing a combined cycle, and the like are actively in progress. Further, research and development of collection techniques of carbon dioxide are in progress.
FIG. 5 is a system diagram of a conventional gas turbine facility 200 in which a part of carbon dioxide produced in a combustor 210 is circulated as a working fluid.
As illustrated in FIG. 5, a combustion gas exhausted from the combustor 210 is guided to a turbine 211 and rotates the turbine 211. Then, the rotation of the turbine 211 drives a power generator 212.
The combustion gas exhausted from the turbine 211 is cooled by passing through a heat exchanger 213. The combustion gas passed through the heat exchanger 213 further passes through a heat exchanger 214. By passing through the heat exchanger 214, water vapor contained in the combustion gas is removed, and the combustion gas becomes a dry carbon dioxide. Here, by passing through the heat exchanger 214, the water vapor condenses into water. The water passes through, for example, a pipe 230 and is discharged to the outside.
The carbon dioxide is pressurized by a compressor 215 and becomes a supercritical fluid. A part of the pressurized carbon dioxide flows into a pipe 232 branched off from a pipe 231. Then, the carbon dioxide flowed into the pipe 232 is regulated in flow rate by a flow rate regulating valve 240, and then guided into a pipe 233 supplying an oxidant. Through the pipe 233, oxygen separated from the atmosphere by an air separating apparatus (not illustrated) flows, as the oxidant. In the pipe 233, a compressor 216 pressurizing the oxidant and a flow rate regulating valve 241 regulating the flow rate of the oxidant are interposed.
A mixed gas composed of the oxidant and the carbon dioxide flows through a pipe 234 and passes through the heat exchanger 213, and is guided to a fuel nozzle 217. Note that the mixed gas obtains heat quantity from the combustion gas exhausted from the turbine 211 and is thereby heated in the heat exchanger 213.
On the other hand, another part of the carbon dioxide pressurized by the compressor 215 is regulated in flow rate by a flow rate regulating valve 243 in the pipe 231 and passes through the heat exchanger 213, and is guided to the combustor 210. The carbon dioxide flowing through the pipe 231 obtains heat quantity from the combustion gas exhausted from the turbine 211 and is thereby heated in the heat exchanger 213. The carbon dioxide guided to the combustor 210 cools, for example, a combustor liner and is then guided to the downstream side of a combustion area in the combustor liner through a dilution hole. The carbon dioxide rotates the turbine 211 together with the combustion gas produced by combustion and therefore functions as a working fluid.
On the other hand, the remaining part of the carbon dioxide pressurized by the compressor 215 flows into a pipe 236 branched off from the pipe 231, and is then exhausted to the outside.
The fuel is regulated in flow rate by a flow rate regulating valve 242 and supplied to the fuel nozzle 217. Then, the fuel is introduced together with the mixed gas guided to the fuel nozzle 217, into the combustion area from the fuel nozzle 217. For example, the fuel is jetted from the center of the fuel nozzle 217, and the mixed gas is jetted from the outer periphery of the fuel. In the combustion area, the fuel and the oxygen react with each other (combust). When the fuel and the oxygen combust, carbon dioxide and water vapor as the combustion gas are produced. The flow rates of the fuel and the oxygen are regulated to have a stoichiometric mixture ratio (theoretical mixture ratio) in a state that they are completely mixed together.
The combustion gas produced in the combustor 210 is introduced into the turbine 211. As described above, a part of the carbon dioxide produced in the combustor 210 circulates in the system.
As the fuel, a hydrocarbon gas fuel or a liquid fuel is used in the above-described conventional gas turbine facility 200 and, for example, use of a coal gasification gas fuel besides those fuels is also under discussion at present.
The coal gasification gas fuel is produced from coal. The coal is large in reserves and is easy and inexpensive to procure. However, when using the coal gasification gas as the fuel, the emission of carbon dioxide increases as compare with the case of using the hydrocarbon gas fuel or the liquid fuel. Hence, if the emission of carbon dioxide can be suppressed, it is beneficial to use the coal gasification gas fuel as the fuel in the gas turbine facility 200.
The coal gasification gas fuel is the one made by gasifying coal in a coal gasification furnace. It takes a predetermined time to bring the gasification furnace into a steady operation state. Therefore, when starting the coal gasification furnace together with the gas turbine facility, it is impossible to obtain the flow rate of the coal gasification gas required for the gas turbine facility at start.
Therefore, in the case of using the coal gasification gas as the fuel, first, a liquid fuel or a hydrocarbon-based gas fuel is used to start the gas turbine in an actual gas turbine facility. Then, after the gasification furnace is brought into the steady operation state, the fuel is switched to the coal gasification gas.
Here, FIG. 6 is a chart schematically illustrating the concentration distributions of fuel and oxygen in the combustor 210 of the conventional gas turbine facility 200. Note that FIG. 6 illustrates the concentration distribution on the left side of a center line (a one-dotted chain line in FIG. 6) of the fuel nozzle 217. The concentration distribution on the right side of the center line is the same as that on the left side of the center line. FIG. 6 illustrates the concentration distributions in a cross section vertical to the center line at a predetermined position on the downstream side of the outlet of the fuel nozzle 217. The concentration distributions illustrated in FIG. 6 schematically illustrate results obtained by numerical analysis.
As illustrated in FIG. 6, the conventional fuel nozzle 217 includes a fuel flow path 290 and a mixed gas flow path 219. These flow paths are divided by cylindrical wall parts 300, 301.
The fuel flow path 290 is provided at the center of the fuel nozzle 217. Into the fuel flow path 290, fuel is introduced via a pipe 235 illustrated in FIG. 5. Then, the fuel is jetted into the combustor 210 from an end portion on the combustor 210 side of the fuel flow path 290.
The mixed gas flow path 291 is, for example, an annular flow path formed on the outer periphery of the fuel flow path 290. Into the mixed gas flow path 291, the mixed gas is introduced via the pipe 234 illustrated in FIG. 5. Then, the mixed gas is jetted into the combustor 210 from an end portion on the combustor 210 side of the mixed gas flow path 291.
In a reaction zone 280, the diffusing oxygen and fuel mix and react with each other. Therefore, as illustrated in FIG. 6, the oxygen concentration and the fuel concentration decrease in the reaction zone 280.
FIG. 7 is a chart illustrating a maximum combustion gas temperature to an equivalence ratio when the mass ratio of oxygen in the mixed gas is changed. In FIG. 7, the maximum combustion gas temperature means an adiabatic flame temperature. FIG. 8 is a chart illustrating a concentration of carbon monoxide to an equivalence ratio when the mass ratio of oxygen in the mixed gas is changed. In FIG. 8, the concentration of carbon monoxide, namely, the vertical axis is indicated by logarithm. The concentration of carbon monoxide is an equilibrium composition value at the adiabatic flame temperature under each condition. Further, the equivalence ratio in FIG. 7 and FIG. 8 is a equivalence ratio when it is assumed that fuel and oxygen are uniformly mixed together.
FIG. 7 and FIG. 8 illustrate results when using the coal gasification gas as the fuel. Note that in the case where the oxygen concentration is 40%, the result when using a natural gas as the fuel is also illustrated. Here, the oxygen concentration means the ratio of the mass of oxygen contained in the mixed gas to the mass of the whole mixed gas.
As illustrated in FIG. 7, the maximum combustion gas temperature increases with an increase in the ratio of oxygen. Further, in comparison between the results of the coal gasification gas and the natural gas, though the ratios of oxygen therein are the same, the maximum combustion gas temperature of the coal gasification gas is higher. This is because the coal gasification gas contains hydrogen and carbon monoxide.
As illustrated in FIG. 8, the concentration of carbon monoxide increases with an increase in the ratio of oxygen. This is caused from an increase in flame temperature with an increase in the ratio of oxygen as illustrated in FIG. 7. More specifically, the increase of carbon monoxide is caused by an increase in flame temperature which accelerates thermal dissociation of carbon dioxide to increase the equilibrium composition value of carbon monoxide.
Further, in comparison between the results of the coal gasification gas and the natural gas, though the ratios of oxygen therein are the same, the concentration of carbon monoxide in the coal gasification gas is higher. As illustrated in FIG. 8, for example, at an equivalence ratio of 1, in the case of using the natural gas, the concentration of carbon monoxide becomes a CO allowable value or less, whereas in the case of using the coal gasification gas, the concentration of carbon monoxide exceeds the CO allowable value.
As described above, in the case of using the coal gasification gas fuel as the fuel in the conventional gas turbine facility 200, the flame temperature increases to increase the emission concentration of carbon monoxide. Hence, in order to decrease the flame temperature, it can be considered to decrease the ratio of oxygen in the mixed gas. However, there is a problem that if the ratio of oxygen in the mixed gas is decreased, a combustion unstable state becomes more likely to occur.